Structure of DNA

• individual nucleotides of DNA are made up of three compounds:
  • deoxyribose (a sugar)
  • phosphate group
  • an organic base belongin to one of two different groups:
    • single-ring bases - cytosine (C) and thymine (T)
    • double-ring bases - adenine (A) and guanine (G)

• made up of two strands of nucleotides (polynucleotides)
• each strand is joined together by hydrogen bonds formed between bases.
• 1953 the structure was worked out by James Watson and Frances Crick  

• Organic bases contain nitrogen and are one of two types:
  • Double-ring structure (adenine and guanine)
  • Single-ring structure (cytosine and thymine)

• Those with a double-ring structure have longer  molecules than those with a single-ring structure 

• the base pairs must always be make up of one of each type, more specifically;
  • Adenine always pairs with thymine by means of 2 hydrogen bonds
  • Guanine always pairs with cytosine by means of 3 hydrogen bonds

• This means A is complementary to T and G is complementary to C.

• The ratio of A and T to G and C varies from species to species. 

• The uprights of phosphate and deoxyribose wind around one another to form a double helix.
• For each complete turn of the helix there are ten base pairs. 

• DNA: the hereditary material responsible for passing genetic information                from cell to cell, generation to generation.
• There are around 3.2 billion base pairs in a typical mammalian cell.
• This means there is almost infinite variety of sequences of bases along the length of a DNA molecule.
• This provides the immense genetic diversity within living organisms.

The DNA molecule is adapted to carry out its functions in a number of ways:

• It is very stable and can pass from generation to generation without change
• Its two strands are joined only by hydrogen bonds, which allow them to separate during DNA replication and protein synthesis.
• It is a large molecule so carries a lot of genetic information.
• The base pairs are within the helical cylinder of the deoxyribose-phosphate "backbone" and so the genetic information, to some extent, is protected from corruption by outside chemical and physical forces.
• DNA depends on the sequence of base pairs that it processes.